A polyester, which includes an aromatic polyester and an aliphatic polyester, is used in a variety of fields for different purposes. The aromatic polyester is mainly used as materials for films, molded articles, and the like. Copolymers of the aromatic polyester with the aliphatic polyester is used as an adhesion agent, a coating material, and the like, and block copolymers are used as a thermoplastic elastomer to produce various kinds of molded articles.
In general, the aliphatic polyester is mainly used as raw materials of a polyurethane, a plasticizer for polyvinyl chloride, medical materials, and the like. Since the aliphatic polyester is characterized by having biodegradability, the aliphatic polyester is expected to be used as fibers, molded articles, sheets, films, and the like by combining one or more kinds of additives as occasion demands.
Conventionally, the aliphatic polyester is produced by a process comprising esterifying directly an aliphatic dicarboxylic acid and an aliphatic glycol, or performing a transesterification reaction between an alkyl ester of an aliphatic dicarboxylic acid and an aliphatic glycol to synthesize a glycol ester and/or a prepolymer thereof and then heating the resulting compound with stirring under a pressure in a range between 0.4 mmHg and 1 mmHg, i.e., high vacuum, for a long perted of time to perform a polycondensation reaction. Note that water is produced as the esterifying reaction proceeds while a glycol is produced as the transesterification reaction proceeds.
However, the aliphatic polyester produced by the above processes has a number-average molecular weight of less than 20,000. In addition, Japanese Official Patent Gazette No. Heisei 5-310898 discloses a process of producing an aliphatic polyester, which comprises performing a deglycol reaction (transestertfication reaction) of an aliphatic dicarboxylic acid and an aliphatic glycol using a catalyst under conditions of a temperature in a range between 180.degree. C. and 230.degree. C., and a pressure in a range between 0.05 mmHg and 0.1 mmHg. Also, Japanese Official Patent Gazette No. Heisei 6-322081 discloses a process of producing an aliphatlc polyester whose number-average molecular weight (Mn) is not less than 5,000, which comprises performing a reaction of an aliphatic dicarboxylic acid and an aliphatic glycol using a catalyst under conditions of a temperatures of 240.degree. C., a pressure of not more than 1 mmHg, and preferably not more than 0.5 mmHg.
However, the above process, in which a polycondensation reaction is performed while heating the aliphatic dicarboxylic acid and aliphatic glycol using catalyst with stirring under high vacuum for a long period of time, readily causes a side reaction and a decomposition reaction of the aliphatic polyester. In other words, the above process produces an abundance of volatile components, that is, by-products produced by the side reaction and low molecular weight compounds such as decomposed components (oligomer) produced by the decomposition reaction. Since the volatile components thus produced have substantially no utility value, the above process can not produce an inexpensive aliphatic polyester.
Further, such an abundance of volatile components readily causes a clog between a reactor (reacting vessel) and a trap. Thus, the reaction must be suspended frequently to remove the volatile components. Moreover, when the volatile components can not be collected completely at the trap, a great amount of the volatile components flows into a vacuum pump. This not only causes a vacuum down (a rise in pressure), but also degrades the performance of the vacuum pump and causes failures or the like. Thus, the above process has a problem that the reaction takes an extremely long period of time and an aliphatic polyester having a predetermined molecular weight can not be produced. In addition, the above process which is carried out under high vacuum demands a high-performance vacuum pump. However, such a high-performance vacuum pump is expensive, and the volatile components easily flows into the same, thereby requiring a considerable effort to maintain the performance thereof. Thus, the above-mentioned conventional process has low industrial feasibility. To eliminate this problem, there is an increasing need for a process capable of producing an aliphatic polyester in industrially efficient manner using a general vacuum pump used for producing a saturated polyester, such as polyethylene terephthalate (PET), whose ultimate vacuum is about 0.5 mmHg, and preferably about 1 mmHg.
On the other hand, a process of producing an aliphatic polyester without using a high-performance pump is known, in which a ring-opening copolymerization reaction of a cyclic acid anhydride and a cyclic ether is performed. For example, Japanese Official Patent Gazette No. Showa 42-26708 proposed a process for producing an aliphatic polyester comprising performing a copolymerization reaction of a cyclic acid anhydride with an alkylene oxide (i.e., cyclic ether) using a catalyst system comprising as one component an organometallic compound of metal selected from groups I to III in the periodic law table.
The process of the above-mentioned Gazette will be explained briefly in the following. In Examples 1 to 6 of the Gazette, a cyclic acid anhydride, an alkylene oxide, a solvent and a catalyst are placed and dissolved in a polymerization tube (a tube for polymerization) to perform a nitrogen substitution. Subsequently, this tube is sealed, and a copolymerization reaction is performed in the sealed tube at 80.degree. C. (Examples 1 to 4 and 6) or 30.degree. C. (Example 5). An inside pressure of the sealed tube is higher than atmospheric pressure owing to a nitrogen gas and vapor of the solvent. The alkylene oxides used are epichlorohydrin in Examples 1 to 3 and propylene oxide in Examples 4 to 6. The cyclic acid anhydrides used are phthalic anhydride in Examples 1 to 4, endo-cis-bicyclo(2,2,1)-5-heptene-2,3-dicarboxylic anhydride in Example 5, and succinic anhydride in Example 6. The catalysts used are triethylaluminum in Examples 1 to 3 and 6, diethylzinc in Example 4, and diethylzinc and water in Example 5.
Among Examples 1 to 6 of the Gazette, an aliphatic polyester is formed only in Example 6 which comprises performing a ring-opening copolymerization reaction of a propylons oxide with a succinic anhydride. However, the aliphatic polyester formed in the Example 6 is inferior in that its melting point is low and thus it has less heat resistance since the cyclic ether used is a propylene oxide.
If a ring-opening copolymerization reaction of a cyclic acid anhydride containing a succinic anhydride as a main component with a cyclic ether containing an ethylene oxide as a main component is performed in the polymerization tube according to the process of the above mentioned Gazette, there is formed a polymer having a melting point lower than a polymer obtained from a polycondensation reaction of a dicarboxylic acid containing a succinic acid as a main component with a glycol containing an ethylene glycol as a main component. The reason is as follows. In the polycondensation reaction, dicarboxylic acid molecules and glycol molecules always bond one by one alternately. In contrast, in the ring-opening copolymerization reaction, not only cyclic acid anhydride molecules and cyclic ether molecules bond one by one alternately, but also the cyclic ether molecules bond with each other to form polyether chains.
In addition, in the process of the above-mentioned Gazette, the reaction takes as long as five to ten days. Thus, not only the copolymerization takes a long time, but also the yield of a resulting polymer is low. A reaction time may be shortened by raising a reaction temperature by introducing a succinic anhydride and an ethylene oxide collectively into the vessel. However, this process causes a defect in that polyether chains are formed and a melting point of a resulting aliphatic polyester is lowered.
The inventors of the present invention formerly proposed a process of producing an aliphatic polyester without using a high-performance vacuum pump in Japanese Official Patent Gazette No. Heidel 6-306154, in which a ring-opening copolymerization reaction of a cyclic acid anhydride containing a succinic anhydride as a main component with a cyclic ether containing an ethylene oxide as a main component using trialkoxyaluminium as a catalyst. However, the number-average molecular weight of a resulting aliphatic polyester is less than 25,000. Thus, the aliphatic polyester processed with molding, for example, into films or sheet, has some industrial problems in that resulting film or sheets are fragile or can not be elongated. Although the aliphatic polyester can have a higher molecular weight if it is reacted with a chain-elongating agent (i.e., cross linking agent), it not only increases the number of steps in a reaction process, but also possibly causes fish eye on a resulting film, thereby making the resulting film industrially disadvantageous. Given these circumstances, there is an increasing need for a process capable of producing an aliphatic polyester having the number-average molecular weight of not less than 20,000 efficiently in a single step.